This invention relates to light sources for backlighting a display and, in particular, to a light source including cascading dyes and a brightness enhancing film (BEF).
Light sources for backlighting displays and other applications often require a relatively full color spectrum. Incandescent lamps have a broad emission spectrum but tend to be reddish, consume too much power, and generate too much heat for most applications. Electroluminescent (EL) lamps rely on the emission of light from phosphors, which generally have a limited spectrum with pronounced peaks. Phosphors emitting different colors can be mixed and cascading dye added, wherein the dye converts light from one phosphor into light of a longer wavelength. Most light emitting diodes produce a very narrow spectrum of light but come in a variety of colors and can be quite bright in small sizes, as desired in displays for hand-held devices.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer includes a phosphor powder or there is a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder emits light in the presence of a strong electric field, using very little current. An EL lamp requires high voltage, alternating current but consumes very little power, even including the current drawn by an inverter for driving an EL lamp.
In many applications, e.g. automotive displays and portable devices such as wrist watches, radios, global positioning systems, and compact disk players, an EL lamp is used for backlighting a liquid crystal display (LCD); e.g., as disclosed in U.S. Pat. Nos. 4,580,877 (Washo) and 5,121,234 (Kucera). The combination is popular because of the uniformity of the back lighting from an EL lamp and the resolution and contrast of the LCD. For full color liquid crystal displays, there is a problem in that the colors available from an EL lamp may not match the LCD.
It is known in the art to that good color performance can be obtained by achieving saturated primary colors and a balanced white point (referring to points on a CIE [Commission Internationale de l""Eclairage] chromaticity diagram). A balanced white point refers to the fact that the eye can be fooled into seeing white from sources that are really not white. For luminous sources viewed directly, such as EL lamps, this is fine. A problem develops if one filters the light in some way, e.g. with a color image in a transparency overlying the EL lamp. The colors in the image look incorrect if a truly white light source is not available. On a CIE diagram, the available light sources must define an area that encloses some white. The saturation of the colors determines how large the area is that encloses white.
In Fundamentals of Liquid-Crystal Displays, Society for Information Display, Jun. 12, 1994, a system is disclosed using fluorescent lamps for backlighting and color filters to match the available light to the colors (nominally blue, green, and red) of the sub-pixels in a color liquid crystal display. Whether or not such a system is effective, it is too bulky for many applications, such as hand-held devices and anywhere that space is at a premium.
U.S. Pat. No. 5,479,328 (Lee et al.) discloses a back lit display using a serpentine fluorescent lamp, a special reflector, and a brightness enhancing film. The Lee et al. patent also discloses that EL backlighting is unsuitable for LCDs because of xe2x80x9cshort lifetime, a low luminous efficiency, and poor color generation. Typical powder EL backlights have a lifetime of only approximately 500 hours. Even newer high-cost powder EL backlights are limited to approximately 1000 hours of use. Moreover, the luminous efficacy of powder EL backlights is totally unacceptable at 0.1 lm/w. In addition, powder EL backlights are limited in color.xe2x80x9d
Brightness enhancing films are commercially available from 3M company and include a plurality of parallel, V-shaped grooves in a transparent film to refract and reflect light. xe2x80x9cCrossedxe2x80x9d films, i.e. films having groove at 90xc2x0 to each other, can increase brightness by up to one hundred percent.
It has long been known in the art to xe2x80x9ccascadexe2x80x9d phosphors, i.e. to use the light emitted by one phosphor to stimulate another phosphor or other material to emit light at a longer wavelength; e.g. see U.S. Pat. No. 3,052,810 (Mash). It is also known to doubly cascade phosphors. U.S. Pat. No. 6,023,371 discloses an EL lamp that emits blue light coated with a layer containing fluorescent dye and fluorescent pigment. In one example, the pigment absorbs blue light and emits green light, while the dye absorbs green light and emits red light.
In view of the foregoing, it is therefore an object of the invention to provide a full spectrum light source from a primary light source having a maximum emission at a wavelength of 550 nm or less.
Another object of the invention is to provide an EL lamp including at least one cascading dye and a brightness enhancing film to produce a full color spectrum.
A further object of the invention is to match a blue or blue-green light source to the color requirements of a liquid crystal display.
Another object of the invention is to provide an improved display system including a liquid crystal back lit by an EL lamp color matched to the liquid crystal display.
A further object of the invention is to provide a full color spectrum from a blue or blue-green light source using at least one cascading dyes and a brightness enhancing film.
The foregoing objects are achieved in this invention wherein a light source producing predominantly blue light is tuned to the transmission bands of color LCD subpixels for optimum light transmission and color separation. The spectral output of the light source has three peaks located in the same areas of the spectrum where LCD color filters transmit most of the light, thereby providing a balanced white. It has been discovered that brightness enhancing films aid in the color conversion by enhancing all colors but particularly red, in addition to increasing the apparent brightness of the light source.